Final answer:
In the compound borax, Na₂B₄O₇, the element with a mass percent composition of 21.49% is Sodium (Na).
Step-by-step explanation:
In the compound borax, Na₂B₄O₇, the element with a mass percent composition of 21.49% is Sodium (Na).
To find the mass percent composition, we need to calculate the molar mass of each element in the compound. The molar mass of Na is 22.99 g/mol, the molar mass of B is 10.81 g/mol, and the molar mass of O is 16.00 g/mol.
Using the formula: Mass percent = (molar mass of element / molar mass of compound) * 100%, we can calculate the mass percent composition of Sodium in borax as: (22.99 g/mol / 201.22 g/mol) * 100% = 11.42%. Since there are two Na atoms in the compound, the total mass percent of Sodium is 11.42% * 2 = 22.84%. Therefore, the correct answer is a) Sodium (Na).